1. Field of the Invention
The invention relates to adaptive control surfaces for aircraft and water craft, and more particularly to such controls having facesheets containing tendons formed from shape memory alloys to form a compliant, controllable surface.
2. Description of the Prior Art
Conventionally, attitude control of aircraft, missiles, undersea craft, and the like is accomplished by means of control surfaces operated from mechanically, hydraulically, or electrically controlled actuators externally coupled to the surfaces. Such systems are typically gimballed about their attachment point and/or employ trailing edge flaps articulated off the control surface. These systems display limited dynamic fluid flow performance manifested in terms of flow separation and reduced coefficient of lift at moderate to high angles of attack.
Prior art control systems are often mechanically complex. Such systems may include lever arms, gears and drives, torsion rods, hydraulic devices, electrical motors, interconnects, and the like. Such elements add weight and complexity to the systems, and often produce maintenance and supply problems.
There is a need for a compliant control surface having no external actuators that can be remotely controlled by electrical signals without requiring transducers, servo mechanisms, or other mechanical and electrical devices.
It is known that shape memory alloys (SMAs) utilize a reversible crystalline phase transformation to recover their original heat-treated shape when heated above a critical transformation temperature range. The recoverable strain can be as high as 8% and can generate recovery stresses as high as 100 ksi. [Miyazaki et al.sup.1 (1989) and Perkins.sup.2 (1975).] SMA wire "tendons", can be used as embedded actuator elements to control the level of facesheet strain in adaptive structural components which utilize sandwich panel construction. As such facesheet strain is varied, the degree of curvature and magnitude of tip deflection will change and can be precisely controlled.
In an exemplary configuration, a core material, such as honeycomb or foam, is bonded to the active facesheet on one side and to a conventional composite facesheet, such as fiberglass/epoxy or graphite/epoxy, on the other side. Electrical resistance heating of the SMA wires is used to produce facesheet contraction (and, therefore, the amount of panel deformation) and a closed-loop strain/displacement sensor feedback loop is used to provide control of the contraction.
Thus, selective control of the heating provides a controlled amount of deformation of the control surface in which these wires are used, whereas the cooling of the SMA wires brings about elongation. One embodiment of the invention making use of this principle may take the form of a control surface constructed to have a single facesheet containing tendons, thus to utilize a flex-biased actuation of the control surface. In other words, contraction of the SMA wires may produce actuation, and the spring action resulting from the stiffness of the panel is used to provide a restoring force when the heating is reduced.
In another implementation utilizing SMA tendons, we may provide a control surface employing an SMA composite facesheet on each side thereof. This construction produces an antagonistic type actuation, for in accordance with this arrangement, the strains in the opposing panels are independently controlled in a differential manner so as to produce curvature of the control surface in either direction from a neutral point.
Significant applications made possible by the use of this invention include adaptive antenna reflector surfaces for balancing thermally induced surface distortions, as well as beam steerage and phasing; mission adaptive aircraft wings for extended range and expanded flight envelope, compliant control fins for marine vehicles such as submarines, torpedoes and ships to reduce noise, turbulence, and system weight, and stators for propulser inflow control and intrapulser flow management. Such surfaces have the ability to improve flow over the control surface minimizing wake interference and improve propulsor inflow.
We are aware of U.S. Pat. No. 5,114,104 granted to Manuel Cincotta et al entitled "ARTICULATED CONTROL SURFACE," which has a control surface shaped by contracting and elongating shape memory alloys embedded within the control surface. An interesting critique of the Cincotta et al patent was contained in the Beauchamp et al U.S. Pat. No. 5,186,420 entitled "ARTICULATED FIN/WING CONTROL SYSTEM," of which Manuel Cincotta, Jr. was a co-inventor. In Column 2 of the Beauchamp et al patent it was stated that the Cincotta et al patent involved a configuration having the disadvantages of low mechanical advantage and severe space limitation. The comment went on to mention that the space limitations result in limiting the number and length of wires which can be used, thus limiting the maximum deflection angle which can be obtained on the control surface.
Another relevant patent is the Roglin et al U.S. Pat. No. 5,150,864 entitled "VARIABLE CAMBER CONTROL OF AIRFOIL," which describes a control surface that is more hinge-like than the instant invention. Roglin's control surface depends on articulation of a trailing edge about a hinge point using an elastomeric strip to maintain compatibility across the control surface. Curvature is confined to a small portion of the foil's coil, and this patentee exhibits no recognition of an antagonistic control arrangement.
It was in an effort to overcome the disadvantages of these and other relatively ineffective prior art devices that the present invention was evolved.